T for transmit. Or is it?

You’re flying along, ATC calls you with a request, you press your push-to-talk (PTT) button, the “T” on your comm radio lights up and you speak. ATC doesn’t hear you. But the “T” is lit? What gives?

Your radio box is just one part of your communications transmitter system. The switches, microphones, cables, connectors and antenna all have to be working properly for you to make clear, readable transmissions.

The Trans-Ind (Transmit Indicator) detects the radio energy from your antenna to let you know your entire communications transmitter system is working. It is small, battery powered and designed to be used during normal flight. Think of it as an oil pressure gauge for your transmitter.

The Trans-Ind has two LEDs — the yellow shows carrier, the blue blinks with voice only if the modulation is greater than 30%, a level at which the audio is clear on a receiver.

“First, is the appropriate amount of radio waves being emitted? and second, are the radio waves modulated with voice?”

If you are emitting at least 10 watts and there is voice modulation, anyone listening within about 60 miles can hear you. But if your radio communications are problematic, McCamant says you should “check both the carrier energy and the modulation.”

Carrier Power

While newer radios show a “T” on their display when the PTT is pushed, the “T” only indicates the button was pushed.

“There is no way for the radio to confirm that it is generating the radio energy or that the energy is coming out of the antenna,” he says. “The transmit monitor provides that confirmation. It detects the energy emitted from the antenna.”

If there is no carrier, or it is intermittent, “start checking the transmit button and wires, then the antenna cable, connectors and the antenna. Many problems are visible, such as a crimped antenna cable or a corroded connector. An ohm meter will confirm operation of the transmit switch.”

Now it’s time to check the power. The Trans-Ind can act as a field strength meter, but you’ll need a friend’s help. Have them press the PTT switch while you walk around the airplane to find the maximum distance the monitor still indicates a carrier.

“A 10-watt radio will cause the indicator to flash up to about 20 feet from the antenna,” he noted.

McCamant advises you map your carrier emissions by noting the distance from the aircraft at the threshold for lighting the carrier indication.

A custom antenna installation or a damaged antenna or ground connection can result in a distorted pattern. McCamant gives an example of a problem behind the right wing. Your radio will operate just fine until the person you are talking to is at your 4-o’clock position.

Modulation

Once you determine you have adequate radio power, it’s time to check audio modulation. Trans-Ind provides indication of 30% modulation. If the blue light blinks when you talk there is enough modulation.

“It is important to understand that for most receivers the loudness of the audio is independent of the radio frequency power. Radio receivers have automatic gain circuits that equalize the radio frequency energy,” says McCamant. “You will be just as loud at 50 miles as you are at one mile. The only thing that changes with distance is the signal to noise ratio. There is more background noise the farther away you are from the transmitter. If people tell you they are having trouble hearing you there is most likely low modulation.”

Intermittent or low modulation should lead you to check the microphone. Make sure it is properly positioned and the pilot is speaking loud enough. Microphone wires also move much of the time and will eventually develop internal breaks and become intermittent. McCamant advises it is good practice to gently pull on the cables and move them around when testing microphone function.

If the microphone, cables, and microphone jacks all check out, it is time to send the radio to the shop for bench tests.

Using a transmit monitor to perform a few tests, you can confirm that your antenna cable, antenna, all the connections from the transmit switch through the antenna connectors are functioning properly.

When used regularly during flight you will have confirmation that the system works every time you use it and clear indication that it is not working as soon as you have a problem.

Be a better informed pilot.

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